This invention relates to a noncontacting optical gauging system and particularly to such a system which illuminates a target surface with a line of light which is evaluated to provide a measure of the surface contour of the target surface.
Optically based gauging systems are presently employed in industry for evaluating the profile shape of workpieces such as turbine blades, gears, helical threads, etc. These devices have inherent advantages over contacting-type mechanical gauges in that they can generally operate at greater speeds and are not subjected to mechanical wear. In one example of an optical gauge according to the prior art, a line or sheet of light is projected onto the object to be characterized. The illuminated portion of the object is viewed with a two-dimensional video camera along an axis at some different angle than the illumination beam. Accordingly, the line of light illuminates a profile of a cross section of the part which is viewed by the camera, just as if the part had been sliced along the light beam. Points nearer to the light source will be illuminated to one side of the field of view of the camera, while further points will be seen illuminated on the other side of the camera's field of view.
The above-described optical gauging techniques have a number of significant limitations. In many instances, it is desirable to evaluate a workpiece surface along a particular cross section, such as perpendicular to the axis of symmetry of a turned workpiece, or along the chord of a turbine engine blade, etc. To characterize such a cross section in one view normally requires the illuminating line of light to be brought in precisely along the specific cross section plane. To evaluate the workpiece contour, the camera views the surface at an angle from the axis of illumination; which produces magnification errors across the surface (the so called "keystone effect") which complicates data processing. Moreover, viewing the plane of interest off axis requires the viewing system to have a depth of field adequate to encompass the depth of interest, which may be difficult to achieve for some workpieces. Illumination systems for such devices also have their own limitations. To provide resolution, the depth of focus of the illuminating line of light must be adequate to encompass the depth of interest. This requirement leads to a greater line width, thus sacrificing accuracy. Finally, those systems are further limited by the pixel resolution of the video image processor.
Alternate gauging approaches such as coordinate measuring machines (CMMs) obtain their accuracy by means of a high precision encoded translation stage. The operational speed of such systems is, however, limited by the use of contact probes which requires the machine to stop and very slowly approach each measurement point. As a hybrid approach, noncontact triangulation probes have been attached to CMMs, but the measurements are still strictly done for selected individual points on the workpiece.
In view of the foregoing, there is a need to provide an optical gauging system which overcomes the depth-of-field and resolution limitations of prior art optical systems and which does not possess magnification variations along the evaluated image which can complicate image processing. It is further desirable to provide such a device which provides rapid gauging time and high measurement accuracy and resolution.